In data transmission systems, each terminal of a transmission link is provided with a clock source for obtaining the timing operations at that terminal. At a transmitting terminal, the local clock generates timing signals to align outgoing data bits in appropriate time slots, assemble the time slots into frames and control the insertion of other signals required in the transmission of the data. At a receiving terminal, the local clock similarly generates timing signals for the recovery of the received data.
To accomplish these functions, the incoming data must be synchronized to the receiving terminal clock. This has been realized by employing a so-called elastic store arrangement. An elastic store typically includes a pair of data stores and successive frames of incoming data are alternately written into the data stores using recovered incoming line timing. The data is alternately read out of the data stores at the receiving terminal timing rate. The data read out is generally phase shifted with respect to data write in so that the write in to one data store occurs simultaneously with the read out of the other data store. However, the recovered line timing to write the received data into the stores for a given line is not necessarily synchronized to the receiving terminal timing and, as a result, more or less data can be written into the stores than is read out of them, causing an overflow or depletion of the stores. To deal with this problem, a "slip" control circuit is used to compare the read and write cycles and when the read cycle effectively drifts or phase shifts to a predetermined extent in either direction relative to the write cycle, the slip control circuit operates on the ready cycle to discard a frame of data or to re-read a frame of data, depending on the relative direction of the phase shift between the read and write cycles. On such elastic store slip control circuit is disclosed in U.S. Pat. No. 3,867,579 issued to J. R. Colton et al on Feb. 18, 1975.
One problem with the prior slip control circuit is that it could not be tested, i.e., exercised, without disrupting transmission of data. Another problem with the prior slip control circuit is that if it failed there were no provisions for indicating the failures and transmission errors would result.